The U.S. launched its first relief airlift to Myanmar on Monday after prolonged negotiations with the country's isolationist junta, which considers Washington its enemy and has restricted international aid to as many as 2 million cyclone victims. The unarmed military C-130 cargo plane, packed with supplies, flew out of the Thai air force base of Utapao for Yangon. Two more air shipments are scheduled to land Tuesday. Myanmar's junta said Sunday the official death toll from the May 3 Cyclone Nargis has jumped to nearly 29,000 with more than 33,000 missing.

But Richard Horsey, a spokesman for U.N. humanitarian operations, said a toll of 100,000 dead or missing was possible based on "reports that we are receiving from our teams in the field from the authorities there."

Though the green light for the U.S. airlift was a positive sign, Horsey said the junta continued to frustrate international efforts to deliver aid. He said clean drinking water, shelter, medical supplies and food are still desperately needed by hundreds of thousands of people in order to prevent widespread starvation and disease.

"It's still a very serious situation. There are up to 2 million people in urgent need of assistance. Assistance is getting through" but not fast enough, he told The Associated Press in Bangkok, Thailand.

He said authorities must allow not only goods to come in urgently but also expedite visas for foreign experts and allowing equipment into the country.

"The authorities of the country need to open up to an international relief effort. There aren't enough boats, trucks, helicopters in the country to run the relief effort of the scale we need," he said. "It's urgent that the authorities do open themselves up."

In the hardest hit Irrawaddy delta, people were surviving in miserable conditions - hundreds crowded into monasteries, where they slept on the floor. Others camped outside, drinking water contaminated by human feces, dead bodies and animal carcasses.

Heavy rains were forecast this week, which would further hinder aid delivery.

"So far we have enough water by collecting rain. But we do not have food anymore," said U Patanyale, the abbot of a monastery in Pyapon town in the delta.

Horsey said the U.N. is getting "a lot of reports" of widespread diarrhea outbreaks in the delta, but not of an epidemic scale. Malaria and dengue could also become a problem.

"But basic shelter, clean water and emergency food will be the thing that if we can get it out fast enough will prevent hopefully these major problems," he said.

The junta has been sharply criticized for its handling of the disaster, from failing to provide adequate warnings about the pending storm to responding slowly to offers of help.

Though international assistance has started trickling in, the few foreign relief workers who have been allowed entry have been restricted to Yangon. Only a handful have succeeded in getting past checkpoints into the worst-affected areas.

The government is also insisting on handling the aid distribution through its feared military, which has ruled this isolated country since 1962.

"The government is very controlling," said U Patanyale. "Those who want to give directly to the victims get into trouble. They have to give to the government or do it secretly. They follow international aid trucks everywhere. They don't want others to take credit."

The Myanmar junta's restrictions on foreign help stems from its strained relations with the international community, especially the West, which has regularly criticized its refusal to allow democracy.

The acceptance of the U.S. relief flight Monday could be "beginning of a long line of assistance from the United States," White House spokesman Gordon Johndroe told reporters in Crawford, Texas over the weekend. "They're going to need our help for a long time."

The plane is carrying 28,000 pounds of supplies, including mosquito nets, blankets and water in an operation dubbed "Joint Task Force Carrying Response."

Lt. Col. Douglas Powell, U.S. Marines spokesman for the operation, said the United States had 11,000 servicemen and four ships in the region for an annual military exercise, Cobra Gold, which could be harnessed to help in the mercy mission.

Highlighting the many challenges ahead, a Red Cross boat carrying rice, drinking water and other goods for more than 1,000 people sank Sunday near hard-hit Bogalay town. All four aid workers on board were safe.

Horsey, the U.N. spokesman, said the boat incident demonstrates the "major logistical bottleneck in getting this (aid) stuff from Yangon out to the people who need it, particularly in the delta."

"A natural disaster is turning into a humanitarian catastrophe of genuinely epic proportions in significant part because of the malign neglect of the regime," said British Foreign Secretary David Miliband.